This invention relates to a large capacity boat lift and, more particularly, to a lift capable of raising and lowering vessels having weights of 75,000 pounds and greater.
Conventional boat lifts for heavy vessels normally employ a plurality of winder driven cables located on respective sides of a boat supporting platform. Pulleys are occasionally utilized to improve the mechanical advantage exhibited by the lift. However, such systems tend to exhibit a number of disadvantages. For one thing, the lift cable when being wound experiences an effect known as reeving wherein stress is exerted unevenly on individual segments of the cable between, for example, the winder, the respective pulleys and the tie-off point of the cable. An especially large force may be exerted on the cable segment that drops from the winder. This may result in premature cable wear and expensive cable replacement. Undesirable wear on the cable is also frequently caused because the cable rubs against the grooves of the winder and/or on the sides of the pulleys. This occurs due to the fleet angle formed between the plane of the winder groove or the pulley groove and the center line of the cable. This too may unduly stress the cable and necessitate premature repairs.
Conventional cable driven boat lifts also tend to permit undesirable longitudinal motion of the boat supporting platform during the raising and lowering operations. This results because the supporting cables effectively shift position longitudinally on the helix of their respective winder grooves as the winders are operated and the cables are wound and unwound. Preferably, the supported vessel should remain as longitudinally stationary as possible during the lifting operation.
It is therefore an object of the present invention to provide an improved large capacity boat lift that is capable of efficiently and effectively raising and lowering virtually all sizes of vessels and which is particularly suited for lifting boats having a weight of 75,000 pounds or more.
It is a further object of this invention to provide a cable operated boat lift that experiences much less cable wear than conventional lifts and which requires less frequent cable repair and replacement.
It is a further object of this invention to provide a cable oriented boat lift that substantially equalizes the tensile stresses on the individual segments of each cable.
It is a further object of this invention to provide a cable operated boat lift that reduces the reeving effect upon the cables.
It is a further object of this invention to provide a cable operated boat lift that permits minimal longitudinal movement of the boat supporting platform during the lifting operation.
This invention results from a realization that a highly efficient large capacity boat lift which exhibits equalized weight distribution on the cables and reduced longitudinal sway may be achieved by operably interconnecting both ends of each lifting cable to opposing grooves on a respective winder such that the cable is raised and lowered at each end.
This invention features a boat lift that includes a pair of axially rotatable winders mounted above the water on respective longitudinal sides of the boat to be lifted. Each of the winders is supported on a respective support structure. A boat supporting platform is disposed between the supporting structures and is operably interconnected to each winder by a respective lifting cable. The cable is attached proximate one of its ends to a first section of the winder and proximate an opposite second end to a spaced apart second section of the winder. Each cable is movably interengaged with an associated set of lower pulleys carried by the platform. An intermediate segment of the cable is also interengaged with the support structure by an upper pulley or otherwise. The winders are axially rotated, typically by respective drive mechanisms to longitudinally drive the respective cables and thereby selectively raise and lower the boat supporting platform. More particularly, when the winder is operated in a first direction to wind its respective cable thereon, the cable is raised to lift the platform. Conversely, when the winder is operated in an opposite direction, the cable is unwound from the winder and extended to lower the platform.
In a preferred embodiment, the cable may be interengaged with its associated lower support structure by means of one or more upper pulleys. The platform may carry at least one set of three axially rotatable lower pulleys on each side of the platform. In such versions, the cable is typically interengaged with a pair of upper pulleys carried by the support structure. The upper and lower pulleys are preferably in the same plane thus eliminating any fleet angle between them. Each upper pulley is located intermediate a respective pair of the lower pulleys carried by the platform. The cable extends alternately ran between the lower and upper pulleys. Typically, the number of lower pulleys exceeds the number of upper pulleys by one. When an even number of lower pulleys and an odd number of upper pulleys are utilized, a centrally located upper pulley remains rotationally stationary during the raising and lowering operations. The cable is one piece. Thus, when the lift is stationary each vertical segment of the cable carries the same load. Equalizing weight distribution prolongs cable life. All of the remaining pulleys rotate as the cable is longitudinally driven and interengages each pulley. Alternatively, when an even number of upper pulleys and an odd lower pulleys are utilized, a centrally located lower pulley (i.e. a lower pulley located between two upper pulleys) remains rotationally stationary during raising and lowering of the platform. All of the remaining pulleys again rotate in interengagement with the longitudinally driven cable. In each version, the stress differential on the individual cable segments is minimized and longitudinal movement of the platform is virtually eliminated.
The winders may be mounted on respective cable beams. Each cable beam may extend between a pair of support posts or pilings. Certain versions of this invention may incorporate at least a pair of winders and associated cables and pulleys on each side of the platform. Each winder may be driven by an associated motor and drive mechanism. Alternatively, a single motor drive mechanism may operate a pair of winders on each side of the lift platform.